Background: Recent research points to high levels of herbal medicine use during pregnancy. The objectives of this study were to elucidate the prevalence and understand the determinants of both the use and self-prescription of herbal medicine during pregnancy. Methods: The study sample was obtained via the Australian Longitudinal Study on Women's Health. Women who were pregnant or who had recently given were invited to complete a subsurvey in 2010 about pregnancy, and complementary and alternative medicine use. Findings: A response rate of 79.2% (n=1,835) was attained and 34.4% (n=588 of 1,835) of the sample were utilizing herbal medicine during pregnancy, of which 77.9% (n=458 of 588) were self-prescribing these products. The women in our study (aged 33-38) were more likely to use herbal medicine if they had anxiety (odds ratio [OR],1.30; 95% CI, 1.02-1.64; p=.031), sleeping problems (OR,1.55; 95% CI, 1.15-2.11; p=.005), or fatigue (OR,1.32; 95% CI, 1.04-1.68; p=.025), but less likely to use herbal medicine if they had nausea (OR,0.71; 95% CI, 0.56-0.91; p=.007). Women were more likely to self-prescribe herbal medicine if they suffered from varicose veins (OR,2.46; 95% CI, 1.04-5.84; p=.041) and less likely to self-prescribe herbal medicine if they suffered from preeclampsia (OR,0.23; 95% CI, 0.81-0.63; p=.005). Women who self-prescribed herbal medicine during pregnancy were also more likely to live in a rural environment (OR,2.22; 95% CI, 1.32-3.73; p=.003). Conclusions: Many Australian women are consuming herbal medicine during pregnancy. The self-prescription of herbal medicine by pregnant women is of particular concern owing to potential safety issues, and it is important that maternity health care providers have an open and nonjudgmental conversation with women about herbal medicine use during pregnancy.